I can do some loaded test of the headphone pre-amp if anyone's interested. I have Ety HF5, Beyer DT250-80, Senn PX-100, Koss KTXPro1, Senn HD201 and Creative EP-630 at hand. I assume I need a 3,5 mm Y splitter with one male and one female connector at the split ends?

Note that while output is 24-bit capable input is 16-bit(oddly, both are 24-bit according to datasheet).

Indeed, the problems seem to be output impedance (-> deviations from flat FR depending on impedance curve of the headphones) and IMD + noise when driving a load. As a source it should work pretty well.

I googled and couldnt find a straight answer to this one: Is there an easy and accurate way to measure output impedance on and headphone output with a multimeter? Depending on where I place the prods at the TRS I get 8, 16, and 32 Ohm on my dedicated amp.

Hmm, from 1.5 kHz to 70 Hz there's a difference of about 4 dB with the PX100 (impedance ranges from about 34 to 60 ohm) so the output impedance seems to be a lot higher. Could be as high as 100 to 200 ohms?

I think you could play some 50-60 Hz sine wave (e.g. use Audacity to generate them) and measure ac voltage without and with load (e.g. 30 ohm resistor). Then calculate Zout = (Rload * (Vnoload - Vload)) / Vload. Not the most accurate way however.

I googled and couldnt find a straight answer to this one: Is there an easy and accurate way to measure output impedance on and headphone output with a multimeter? Depending on where I place the prods at the TRS I get 8, 16, and 32 Ohm on my dedicated amp.

You have to measure the voltage with 2 known resistances and then solve for the output impedance.

The output impedance isn't just tied to the DAC chip, it will depend on the implementation on your PC / laptop.

An easier way to measure output impedance is to play and record a sine wave at 1 kHz, without a load, and with a load of known impedance (like headphones with a stereo splitter). Determine the RMS value of both recordings (e.g. with "sox file.wav -n stat -rms"), then use the following formula:

Zout = (Zhp * (RMShigh - RMSlow)) / RMSlow

You can adjust that value by taking into account your ADC's input impedance, if you know it:

Zout = 1 / ((1 / Zout) - (1 / Zin))

I've managed to make reliable measurements that way (verified by comparing my results to other results available online). Don't ask me about the math, I got it from stv014, but I know it works.

According to the spec sheet the amplified output impedance for ALC889 is 2Ohms. Line out is 200Ohms.

The datasheet also recommends (in the "Application Circuits" section) 75 Ω serial resistors (and also 100 uF capacitors) on the headphone output, so an implementation that follows the reference design would have an output impedance of 77 Ω.

While I'm at it, I discovered a way to circumvent a problem that sometimes arises while testing different audio interfaces mixed and matched.

In this test I was never able to get a recording level of greater than -12 dB, which the Rightmark program had problems with. I simply had the Rightmark program save the wave file that it recorded, normalized it with Audacity, and then analyzed it. Since the noise level of the AP 24192 card is about 110 db below its normal FS input, the test was run under a condition where this was effectively 98 dB below the tests, and so errors due to the circumvention were still well below the dynnamic range that was observed.

ALC887 line output (rear jack) on an ASUS motherboard. The front panel is not connected, because it is used by a sound card, so I cannot test the headphone output, but I measured it earlier to have an output impedance of slightly more than 75 Ω.

The output impedance of ~200 Ω is consistent with the datasheet. There are apparently also 10 uF capacitors on the output (calculated from the output impedance and the frequency at which the phase is -45 degrees).